Global Chaotic Mixing on Isentropic Surfaces

Abstract

Abstract The general nature of two-dimensional mixing on isentropic surfaces in the troposphere has been investigated. The daily time series of isentropic winds is obtained from a global general circulation model and is used to drive a high-resolution fully Lagrangian passive tracer model. Results are compared with the extremes of chaotic mixing by organized waves on the one hand and classical diffusion on the other and are found to lie in the middle ground. Advection by planetary-and synoptic-scale eddies generates small scales from an initially smooth tracer field exponentially fast, but given a modest degree of smoothing the tracer field evolving from a localized release rapidly attains the form of an algebraically spreading cloud. The zonal size of the cloud increases linearly with time (superdiffusively), owing to the systematic shear in the extratropical zonal jets, while the meridional spread has the square-root-of-time increase characteristic of classical diffusion. It is argued, however, that the...